Data from: Dancing together and separate again: gymnosperms exhibit frequent changes of fundamental 5S and 35S rRNA genes (rDNA) organisation

In higher eukaryotes, the 5S rRNA genes occur in tandem units and are
arranged either separately (S-type arrangement) or linked to other
repeated genes, in most cases to rDNA locus encoding 18S–5.8S–26S genes
(L-type arrangement). Here we used Southern blot hybridisation, PCR and
sequencing approaches to analyse genomic organisation of rRNA genes in all
large gymnosperm groups, including Coniferales, Ginkgoales, Gnetales and
Cycadales. The data are provided for 27 species (21 genera). The 5S units
linked to the 35S rDNA units occur in some but not all Gnetales,
Coniferales and in Ginkgo (~30% of the species analysed), while the
remaining exhibit separate organisation. The linked 5S rRNA genes may
occur as single-copy insertions or as short tandems embedded in the
26S–18S rDNA intergenic spacer (IGS). The 5S transcript may be encoded by
the same (Ginkgo, Ephedra) or opposite (Podocarpus) DNA strand as the
18S–5.8S–26S genes. In addition, pseudogenised 5S copies were also found
in some IGS types. Both L- and S-type units have been largely homogenised
across the genomes. Phylogenetic relationships based on the comparison of
5S coding sequences suggest that the 5S genes independently inserted IGS
at least three times in the course of gymnosperm evolution. Frequent
transpositions and rearrangements of basic units indicate relatively
relaxed selection pressures imposed on genomic organisation of 5S genes in
plants.